FFT Frame Scope (DSP Blockset)

Compute and display the frequency content of a frame-based input.

Library

DSP Sinks

Description

The FFT Frame Scope block displays the magnitude of the FFT of the input, which is assumed to be a frame of sequential time-samples.

The FFT length parameter, N_fft, specifies the number of samples on which to perform the FFT. A value of -1 instructs the block to use the input frame size as the FFT size, in which case you can also set the Sample time of original time-series parameter to -1 (see below). Otherwise, the block zero pads or truncates the input to N_fft before computing the FFT.

In order to correctly scale the frequency axis (i.e., to determine the frequencies against which the transformed input data should be plotted), the block needs to know the actual sample period of the time-domain sequence. The Sample time of original time-series parameter allows you to specify this in two different ways:

Auto-Detect from Input Sample Period. A value of -1 for this parameter instructs the block to compute the frequency data from the sample period of the input. This parameter setting is appropriate if the FFT is computed on the same number of points as are contained in the time-domain input (i.e., no zero-padding or truncation). This is only true when the FFT size parameter is set to the size of the input (or, equivalently, to -1).

The auto-detect mode also makes the following two assumptions:

The sample period of the original time-domain signal in the simulation is equal to the period at which the physical signal was actually sampled. For example, the mtlb signal imported from the workspace in the model below has an actual sample period of 1/Fs = 1/7418.

Although the Sample time parameter in the Signal From Workspace block can legitimately be set to any value, for the auto-detect mode to compute the correct frequency scaling, the Sample time parameter must be set to the original sample period of 1/7418.

Consecutive frames of the time-domain signal do not overlap each other; that is, a particular sample of the time-domain signal does not appear in more that one sequential frame.

Enter the Appropriate Time-Domain Sample Period. Enter the sample period of the time-series, T_s. This is necessary when the FFT is computed on a different number of points (more or fewer) than are contained in the time-domain input. When the FFT length parameter is set to a value other than the frame size of the input, the block either zero-pads or truncates the input before performing the FFT, and cannot automatically compute the original time-domain sample period.

You also need to explicitly specify the time-series' actual sample period when either of the assumptions listed above for the auto-detect mode is not valid.

The Frequency units parameter specifies whether the frequency axis values should be in units of Hertz or rads/sec, and the Frequency range parameter specifies the range of frequencies over which the magnitudes in the input should be plotted. The available options are [0..Fs/2], [-Fs/2..Fs/2], and [0..Fs], where F_s is the time-domain signal's actual sample frequency (F_s/2 is the Nyquist frequency). If the Frequency units parameter specifies Hertz, the spacing between frequency points is 1/(N_fftT_s). For Frequency units of rads/sec, the spacing between frequency points is 2

/(N_fftT_s).

Note that all of the FFT-based blocks in the DSP Blockset, including those in the Power Spectrum Estimation library, compute the FFT at frequencies in the range [0,F_s). The Frequency range parameter controls only the displayed range of the signal.

The scope updates the display for each new input. The input can be an M-by-N frame matrix, where each of the N frames in the matrix contains M sequential time samples from an independent signal channel. The Number of input channels parameter specifies the number of independent channels (columns), N, in the matrix. The block overlays a separate FFT plot for each of the N independent channels in the input.

For information about the scope window, as well as the Axis properties and Line properties panels in the dialog box, see the reference page for the Time Frame Scope block.

Dialog Box

FFT length: The number of samples on which to perform the FFT. If the FFT length differs from the size of the input vector, the data is zero-padded or truncated as needed. A value of -1 sets the FFT length to the input frame size.
Frequency units: The frequency units for the x-axis, Hertz or rads/sec.
Frequency range: The frequency range over which to plot the data, [0..Fs/2], [-Fs/2..Fs/2], or [0..Fs], where F_s is the sample frequency of the original time-domain signal, 1/T_s.
Sample time of original time series: The sample period, T_s, of the original time-domain signal. Set to -1 to auto-detect the signal sample period from the frame period and frame size of the block input (use only when the FFT size is equal to the size of the input or -1).
Y-Axis title: The text to be displayed to the left of the y-axis.
Amplitude scaling: The scaling for the y-axis, dB or Magnitude.
Number of input channels: The number of channels (columns) in the input matrix.
Axis properties: Select to expose the Axis Properties panel. See Time Frame Scope for more information.
Line properties: Select to expose the Line Properties panel. See Time Frame Scope for more information.